Ferrous metals are often used in environments where the surface is exposed to abrasive and corrosive elements. For example, in drilling oil wells, the wear surface of downhole earth boring equipment such as rotary bits, drilling tools and the components thereof are exposed to the highly abrasive activity of drilling in an environment laden with corrosive elements, often at elevated temperatures.
The desire to create a hard and durable outer surface to resist the abrasion and corrosion, while maintaining a ductile interior, has led to the science of case hardening of metals. In case hardening, a process is used which will produce a hard outer surface or case on the metal while permitting the core of the metal within to remain relatively soft and ductile when subjected to normal ferrous metal thermal treatment.
One common process of case hardening is carburizing. Carburizing is particularly effective with low carbon and alloy steels and permits selective surface hardening of the metals. Carburizing consists of the process of diffusing nascent carbon into a ferrous surface at an elevated temperature. The depth of penetration of the nascent carbon depends upon the temperature and length of time the ferrous material is exposed to a source of the carbon. The carbon can be supplied to the metal by a number of techniques. The ferrous surface can be exposed to a carbon rich gas or liquid. The material can also be surrounded by solid carburizing compounds to perform a pack carburization.
In addition to carburization, ferrous metals have been treated with other materials to provide other properties as required. For example, the diffusion of boron into a ferrous metal provides an outer case having greater resistance to corrosion and wear than that supplied by carburization. A boron case will also provide a lower case surface coefficient of friction than that provided by a carburized case.
A combination of these desirable properties has been achieved by both carburizing and boronizing a ferrous metal. In the past, a two stage process has been used to diffuse the carbon and boron into a ferrous metal. The metal is initially heated and exposed to nascent carbon to permit the carbon to diffuse into the surface of the metal to form a carbide substrate. The metal is subsequently cooled and reheated in the presence of boron. The boron diffuses into the metal surface to form a boron rich layer superimposed over the carbon rich layer.
U.S. Pat. No. 3,923,348 issued to Peck on Dec. 2, 1975 discloses a technique for hardening a bushing. The bushing has a ferrite and martensite core. A carbon diffused layer is provided on the ferrous substrate followed by a boron case. U.S. Pat. No. 3,922,038 to Scales issued on Nov. 25, 1975 discloses a treatment for ferrous substrate. In this technique, the ferrous surface is initially carburized. The ferrous material, after carburizing, is then boronized. Finally, the material is hardened and tempered. U.S. Pat. No. 4,188,242 to Scales issued on Feb. 12, 1980 discloses a method of carburizing and boronizing steel with subsequent hardening and tempering.
Several shortcomings have been noted in the processes disclosed in these patents and other known processes. Since the carbon diffused into the metal in the initial carburizing stage inhibits boron diffusion, only thin boride cases are possible, in the range of 0.003 inches to 0.006 inches. In addition, the diffusion of boron into the metal has a tendency to diffuse the carbon deeper into the metal to give rise to potentially undesirable additional carbon diffusion. In addition, the boron surface layer is found to be extremely brittle and subject to cracking. While this problem can be somewhat alleviated by placing the boron layer in compression, this limits the applications of the materials treated by these processes. To avoid cracking, the boron case thickness must be maintained within a specified range, again restricting the versatility of the materials treated by the process. Therefore, a need exists for an improved process which combines the advantages of the various materials and overcomes the shortcomings of the prior processes.